The present invention relates generally to integrated injection logic (I.sup.2 L) and more specifically to an improved substrate fed logic (SFL) semiconductor structure.
A typical example of a substrate fed logic semiconductor structure of the prior art is illustrated in FIG. 3b of the U.S. Pat. No. 3,916,218. The structure consists of four layers alternating, namely P1, N1, P2 and N2. The four layers form the complementary, vertical, inversely operated bipolar transistors. This structure has technical drawbacks. For example, by using an epitaxial layer P2 as the base of the NPN transistor, the base width will vary with epitaxial thickness and taper control. This is undesirable in a neutron radiation hardened circuit. Neutron radiation introduces recombination sites in the base and the gain becomes limited by base transport factor vs pre-radiation gain domination by emitter efficiency. Since the base transport factor is strongly related to the base width, the post neutron gains will vary with the epitaxial thickness control. This unpredictable variation in post neutron gain makes the semiconductor structure of U.S. Pat. No. 3,916,218 undesirable for a radiation hardened design. Similarly, P2 has a constant doping profile and hence does not have either an aiding or hindering electric field affecting upwards gain. Similarly, by the use of an epitaxial P2 layer, the P and N.sup.+ diffused regions must be diffused down to and through region P2, respectively. This increases the total diffusion time and, consequently, produces undesirable epitaxial junction movement, which is a function of the square root of the product of diffusion time and temperature.
To increase the inverse gain and f.sub.T of integrated injection logic structures, the prior art has suggested upward diffusing the base of the NPN vertical transistor. The updiffused base structure is discussed in "Integrated Injection Logic" in research disclosure No. 48, Page 67 by Industrial Opportunities Limited of Homewell, England. The structure is formed by outward diffusion of boron through a top epitaxial layer to the top of the said epitaxial layer. The resulting structural has an NPN base width which varies as the top epitaxial thickness varies. This produces the same undesirable results discussed in for U.S. Pat. No. 3,916,218.
The I.sup.2 L structures of the prior art generally are not radiation hardened to three simultaneous environments, namely total gamma dose hardness, neutron hardness, and dose rate hardness. Thus there exist a need for an I.sup.2 L structure which is radiation hardened to three simultaneous environments.